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Image Darkening in MC h264 encoding

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Stefan Day
Image Darkening in MC h264 encoding
on Apr 2, 2010 at 5:42:34 am

Hi,

Im encoding media clips from various original formats (4:3ntscDV to proress422 1080i60) into MC h264 mp4 videos for streaming. I export my final edits as FCP QT movies, self-contained and then drop them into squeeze.

Consistently the resulting image is substantially darker that the original. Why is this? Is it a set value that is uniformly correctable?

Stefan


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Chris Blair
Re: Image Darkening in MC h264 encoding
on Apr 4, 2010 at 2:42:28 pm

It shouldn't be. Main Concept's codec doesn't do anything that should cause that. Are you looking at the encoded video using the same media player on the same monitor you're looking at the Final Cut sequences?

That's the only way to do a fair comparison...by making sure everything is equal in the playback, meaning the monitor and the media player.

Chris Blair
Magnetic Image, Inc.
Evansville, IN
http://www.videomi.com


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Larry Little
Re: Image Darkening in MC h264 encoding
on Apr 10, 2010 at 10:02:04 am

I've been struggling with the exact same issue using the MainConcept h.264 encoder in Squeeze 6 -- the resulting image is substantially darker than the input source. I'm not sure if it's a Squeeze issue, a MainConcept issue, or an issue with the QTRef files from Avid MC that I'm using, but I do find other people talking about this on the web, so it's not unique to me (or Stephen, who started this thread.) It's not subtle -- it's a VERY obvious shift.

In order to do the least correction inside of Squeeze, I found that I had to start with QTRefs that have 601 levels (16-235.) These end up being shifted closer to RGB levels on the encoded file. In other words, the input source is 16-235 (QTRef file output from Avid Media Composer), but the encoded output ends up much closer to 0-255.

Unfortunately, it's far from an exact 601-to-RGB level shift, so I find that I still have to do some extra correction in Squeeze. If I drop gamma to -11, it's "relatively" close, but it would be nice to get it closer (which is what led me to search for other people's solutions to this, and in turn to find this thread.) I've tried all sorts of combinations of corrections in Squeeze using varying levels of Gamma, Brightness, Contrast, Black Restore, and White Restore. Any given combination of settings will look better with certain scenes vs others (i.e. dark vs light scenes, etc.), but in the end, it seems like simply using -11 Gamma in Squeeze looks generally pretty good overall across a variety of scenes. It's just a tad more contrasty and just a bit lighter overall compared to the original, but my other more complicated setting combinations come with other drawbacks, so for the moment I'm just using -11 gamma.

On another unfortunate note, none of other reports of this offer any solid solution other than to fix it via trial and error. This includes at least one post on the Sorenson forum reporting this same issue. I'd be very interested in hearing any other information on this issue, or other setting combinations that people have used to resolve this.

Thanks,

Larry

PS. Note that you cannot use the "Preview" window in Squeeze 6 to dial in the correction settings because the preview image is VASTLY different than the encoded output -- it's not even close. For any sort of color or luminance correction, the preview window in Squeeze is quite literally useless. This has also been reported on the Squeeze forum, and I reported it via the phone support as well.


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Larry Little
Re: Image Darkening in MC h264 encoding
on Apr 10, 2010 at 12:24:06 pm

Just to clarify, the -11 gamma setting I mentioned above is in addition to the fact that I'm using a 601 (16-235) QTRef source file, which expands when it's encoded due to the Squeeze MC encoder issue. The effect of the luminance expansion AND the Gamma setting result in a "ballpark" correct result.

Larry



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Larry Little
Re: Image Darkening in MC h264 encoding
on Apr 11, 2010 at 6:01:41 am

I should also add that this is using Quicktime to view the files, and comparing the original QTRef to the output.

Larry



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